He wears a St. Hermelin High School uniform. A distinctive trademark incorporated into his uniform is a scarf with the number 1 printed on it. This symbolizes his desire to become the number one man in Japan.

In Revelations: Persona, his hair turned blond and he had lighter skin.

In Persona 2: Eternal Punishment, he wears a black motorcycle outfit with number 1 printed on the back, retaining his scarf, goggles, and he's mostly seen with his white motorcycle helmet on.

In Persona 3, Trish mentions that he can be identified by all the #1 designs on his clothing.

Kei was forced to join in on the demon summoning game "Persona," which was started by a couple of his classmates. Kei chooses to just watch, as he doesn't really care about the result and feels that everyone was just feeding Masao and Hidehiko's stupid squabbles.

Kei is the most logical member of the group, this being both a good and a bad thing. At two points in the game, Kei suggests to let two innocent people die in order to save everyone else, the professor in charge of managing the DEVA System, who trapped himself in the machine along with Kandori to stop him from destroying reality and the true Maki Sonomura found after the Ideal Maki escapes from the party. However, these extreme choices are usually seen negatively by the rest of the party, with the latter leading to Masao punching Kei down.

Even with his seemingly collected and oftentimes stiff personality, Kei has a solitary weakness, and it is none other than the theme song often played at the chain of pharmacies owned by the Tadashi Family. This first manifested back in high school during the investigation on SEBEC, and haunts him up to adulthood. Kei feels that the silly lyrics hypnotize him, which invariably results in a total loss of composure.

Kei usually uses the term "shounen" (young lad) despite oftentimes being the same age or younger than the person he is talking to. His upbringing could be a factor that brought about this unusual habit.

While his parents are often away on business, Kei is looked after by Yamaoka, the Nanjo family butler. Since then he seems to have grown up under Yamaoka's watch, he considers him as more of a father figure than a servant.

In Persona 2: Eternal Punishment, Kei is very verbose with his victory cries, often complimenting his "Nanjo-style swordsmanship" and spouting out philosophical proverbs that suit the occasion.

In Persona 3, Trish of the television program Who's Who describes him as a man in his twenties who is proud of his intelligence.

Kei was forced to join into the demon summoning game "Persona," which was started by a couple of his classmates, Masao Inaba and Hidehiko Uesugi. The two boys were holding a bet to see whether the dubious game would call on demons, spirits or other weird things, with the loser forced to treat everyone at Peace Diner.

For his part, Kei chooses to just watch, as he doesn't really care about the result and feels that everyone was just feeding Masao and Hidehiko's stupid squabbles. Regardless, he also witnessed the spirit of a sobbing little girl, and was incapacitated by bolts of lightning which sent him to the realm bordering consciousness and death where he received his Persona from Philemon.

Being heir of a wealthy family, Kei at some point met with Takahisa Kandori, head of Saeki Electronics & Biological Energy Corporation, and the game's primary villain; whom he takes instant dislike to from the beginning. During the SEBEC raid in the first part of the game, just before fighting Kandori and his men, Kei, in a moment of rage attacks Kandori with his Persona, ultimately missing. In retaliation, Kandori attacks the party with his Persona "Nyarlathotep" and leaves his henchmen Takeda to fight for him. From there on until the last confrontation against Kandori, Kei takes an attitude of anger and pity towards him, deeming him as weak, even to the point of taunting him and provoking him to battle.

According to the epilogue, after the events of the game, as part of his executive training, he took a job incognito (in disguise) as a salesman at a Nanjo Group subsidiary. He finds out how firsthand how cruel the business world can be. After seeing how the rest of the world lived, he slowly became mellower. His arrogance from the past has turned into compassion for all people. Some have spotted him sleeping on the train, wearing a tie embroidered with the phrase "No.1."

In Revelations: Persona, he was renamed to Nate Trinity, and he was the heir to the Trinity Family Trust instead of the Nanjo Group.

"Yukino's friend, briefly returned from his studies in England. In training to become the number one man in Japan."

—His introduction

Kei later makes an appearance in both Innocent Sin and Eternal Punishment. Prior to the game's events, Kei was studying abroad in England. In Innocent Sin, he is a supporting character who appears at the Abandoned Factory in Konan, drawn to rumors that a person resembling Takahisa Kandori is in Sumaru City (the person in question turns out to be Reiji Kido, who isn't too happy about being mistaken as his hated half-brother). When you talk to him, Kei will give Tatsuya Suou his ultimate weapon. During the invasion of the Fuhrer's Last Battalion, he joins his former classmates at Sevens to repel the Nazi invasion if Principal Hanya is said to have died earlier in the game.

In Eternal Punishment, he is a playable character. Sneak mentions that there are two people (a man and a woman) investigating the New World Order. If Maya spreads the rumor that the person with information on the NWO is a man, you will get Kei to join. He now uses katanas like Tatsuya, and when using his demon contacts, Kei still mentions his beloved butler Yamaoka, and how he intends to honor the promise he made years ago to become the #1 man in Japan as the latter watches over him from above.

Kei will take the group to a Science Lab run by Tatsuzou Sudou and funded by the Nanjo Group, where youths who allegedly performed the Joker Curse are kidnapped and drained of Kegare. The party will run into Kei's old classmates Reiji Kido and Maki Sonomura around the sewer entrance, assisting in their covert infiltration by working the floodgates. Kei will rejoin the rest of his former classmates later in the game (with Tatsuya taking his place in Maya's party), fighting off the New World Order soldiers that are trying to take over the city.

Kei does not appear in person in Persona 3; however, his company was mentioned by Takeharu Kirijo. Takeharu explained that at one point the Kirijo Group was part of the Nanjo Group but one day they separated from each other. Kei is also a guest on Who's Who. Trish describes him as a man in his 20's who is proud of his intelligence. She also mentions that he can be identified by all the #1 designs on his clothing.

Known as Nate, Nanjo is a C-Rank BattleNet opponent. Lv. 16, he is described as one who learns from his defeats, whose goal is to aim for the top. Defeating him 10 times has him give a unique Basilisk named Dethlisk, defeating him 20 times earns a Sizzard.

Kei can develop a demon contact with Baofu called "Song of a Baseball Fan," which involves them listening to a pre-recorded radio broadcast and getting into a heated debate over which of their chosen teams is the best, mirroring the typical discourses among the Japanese baseball fandom, especially regarding the long-standing rivalry between the Tokyo Giants and the Hanshin Tigers.

During the fight against Takeda, Kei will summon his Persona and attempt to use Hama on Kandori even if his equipped Persona doesn't have the spell.